Nonlinear optics is the study of how intense laser light interacts with optical materials. Nonlinear optical processes typically generate coherent photons with new frequencies and wavelengths. Hence one benefit is access to spectra that are not available using conventional lasers.
Bulk nonlinear crystals (generally, crystals that are hundreds of micrometers to several millimeters in their spatial dimensions) have been widely used for nonlinear optical processes such as second harmonic generation, third harmonic generation, sum frequency generation, difference frequency generation, and the like. Despite the wide availability of nonlinear crystals, there are demands for more compact structures to realize high efficiency nonlinear processes. Moreover, traditional nonlinear optical processes using bulk crystals require phase matching between the fundamental frequencies and new generated frequencies.
Generally, the use of uniaxial or biaxial bulk nonlinear crystals, or the application of quasi-phase matching techniques, is required in order for the interacting optical fields to meet the strict phase-matching conditions.
Recently, however, advances in nanostructured optical materials, plasmonics, and metasurfaces have enabled nonlinear optical processes that do not depend on phase matching. These approaches create tight confinement and large resonant enhancement of electromagnetic fields, which generate much higher nonlinear efficiencies than in the constituent materials.
Moreover, metasurfaces comprising arrays of Mie dielectric resonators have attracted recent attention at optical frequencies due to their much lower loss compared with their metallic counterparts. In particular, silicon has been used extensively as the constituent material for all-dielectric metamaterials that have been used for a variety of applications including high efficiency Huygens metasurfaces, beam steering, ultra-thin waveplates, zero-index directional emission and polarization insensitive holograms.
In the last few years, it was realized that dielectric nanoresonators can also be used to greatly enhance nonlinear optical phenomena, due to the largely enhanced electromagnetic fields inside the resonators and the larger mode volume. However, due to the centrosymmetric crystal structure of silicon, second-order nonlinear optical phenomena were not observed in silicon-based metasurfaces.
Therefore, there has been a need for dielectric metasurfaces based upon other materials that exhibit an intrinsic second order nonlinear susceptibility (χ(2)) for fuller exploitation of this approach for enhanced harmonic generation and other second-order nonlinear phenomena.